The presently disclosed subject matter relates to a biological signal measuring system, and more particularly to a system for, after pressing living tissue, identifying a time period for filling the living tissue with blood (hereinafter, the time period is referred to as the blood fill time), based on a biological signal acquired from a subject that is an example of the living body. The presently disclosed subject matter relates also to a biological signal measuring apparatus which is to be used in the system.
Measurement of the blood fill time is a technique which is used in the field of emergency medicine in order to determine necessity/unnecessity of transfusion or the priority in a scene of triage. Specifically, a medical person pressurizes living tissue of the subject, such as a fingertip, and visually checks a change of the color of the skin after the pressurization is released. If the color returns to the original color within two seconds, it is determined that the subject is in a normal condition. However, the technique where living tissue is pressed by the hand, and a change of the color of the skin is visually checked has low quantitatively. Moreover, an error due to the measuring person easily occurs.
Therefore, a method has been proposed in which a pulse oximeter is used in measurement of the blood fill time (for example, see JP-A-2012-115640). Specifically, a light beam of a wavelength which allows the light beam to be absorbed into blood is incident on living tissue such as a fingertip, and the intensity of the light beam which is transmitted through the living tissue is measured (hereinafter, the intensity is referred to as the transmitted light intensity). When a portion of the living tissue is pressurized, blood is evacuated from the portion, and therefore the transmitted light intensity is increased. When the pressurization is released, the portion is filled with blood, and therefore the transmitted light intensity is decreased. The blood fill time is identified based on the time period which elapses after the release of the pressurization until the transmitted light intensity returns to the original level.
A difference in intensity of pressurization on living tissue may affect results of identifications of the blood fill time. FIGS. 6A and 6B show results of plural identifications of the blood fill time which were performed while changing the pressurization intensity. FIG. 6A shows results in the case where, even where pressurization was released, the transmitted light intensity did not return to the original level. FIG. 6B shows results in the case where, as the measurement was repeated, the reference value of the transmitted light intensity was gradually reduced.
It is supposed that the results shown in FIG. 6A were caused by the phenomenon that deformed tissue is made to hardly return to the original state by strong or repeated pressurization, and the blood portion requires a long time for having the original thickness. It is supposed that the results shown in FIG. 6B were caused by the phenomenon that, conversely, the blood flow is promoted by pressurization, and the thickness of the blood portion is increased. In both the cases, as far as the blood fill time is identified based on the time period which elapses until the transmitted light intensity returns to the state that is attained before pressurization, the results inevitably involve an error. However, pressurization on living tissue is manually performed by a medical person, and therefore it is difficult to always perform pressurization at a constant strength.